1. Field of the Invention
The present invention relates generally to a liquid crystal light valve projector assembly including a liquid crystal light valve, a cathode ray tube (CRT) or other such means for producing modulated writing light representative of a given image which is applied to the light valve, and projection optics responding to the light valve for projecting a correspondingly modulated projection light beam onto a projection plane for visually displaying the given image. The present invention relates more particularly to a fiber optic plate arrangement which is provided for coupling the CRT or other such light-writing means to the light valve and which is specifically designed to eliminate from the projected image a faint honeycomb pattern known as the "chicken wire effect".
2. Summary of the Prior Art
A typical liquid crystal light valve projector assembly of the prior art is described in an article entitled FULL-COLOR SINGLE-PROJECTION-LENS LIQUID-CRYSTAL LIGHT-VALVE PROJECTOR by Arno G. Ledebuhr, SID 86 DIGEST, pages 379-382. The projector illustrated in this article is reproduced herein as FIG. 1 and is generally designated by the reference numeral 10. Projector 10 is shown including (1) a liquid crystal light valve 12, (2) a cathode ray tube 14 for producing modulated writing light which is representative of a given image and which when written (actually projected) onto light valve 12 causes the latter to modulate a projection light beam 15 in a corresponding manner, and (3) projection optics 16 and a light source 18 for providing the projection light beam 15 and for projecting this correspondingly modulated projection light beam onto a projection screen 20 for visually displaying the given image. As illustrated in FIG. 1, the projection optics includes, among other components, a polarizing (and beam splitting) prism 22 and a projection lens 24.
In addition to these components, projector 10 shown in FIG. 1 includes a fiber optic face plate arrangement 26 for coupling the CRT to the light-writing side of light valve 12. Fiber optic face plate arrangement 26 is shown including two separate and distinct fiber optic face plates 27 and 28 which are themselves optically coupled to one another by means of a suitable index matching fluid, for example oil (not shown). The upstream fiber optic face plate 27 includes a light entering end face which is mounted to and forms part of the screen of CRT 14 and, at the same time, downstream fiber optic face plate 28 includes a light exiting end face mounted to the light-writing side of light valve 12. As seen in FIG. 1, the otherwise free sides of these two fiber optic face plates are in confronting relationship to one another.
Having described fiber optic plate arrangement 26 generally, attention is now directed to a more detailed description of each of the fiber optic face plates forming part of this arrangement. In particular, FIGS. 2 and 3 are enlarged and extra enlarged views, respectively, of a section of fiber optic face plate 28. As seen in FIG. 2, this face plate is comprised of an array of lengthwise adjacent optical fibers 30 having opposite end faces lying on opposite sides of the face plate. In addition to these optical fibers 30, the face plate includes a number of light absorbing fibers 32 spaced apart from one another and surrounded by adjacent optical fibers 30, as illustrated in FIG. 2.
As can be seen in FIG. 3, each optical fiber 30 is comprised of a central core 34, a cladding layer 36 surrounding the core, and a layer of flux material 38 which surrounds the cladding layer and which bonds the cladding layer and core to adjacent optical fibers. In a typical embodiment of face plate 28, the cores, cladding, and flux making up optical fibers 30 are formed of glasses of differing hardness and indices of refraction, depending on the particular application. In this same embodiment, the light absorbing fibers 32 are typically formed of statistical extramural absorption material (EMA). It should be noted that of the components recited, the light absorbing fibers 32 are physically the hardest, followed by cores 34, and then cladding layers 36. The flux material 38 is physically the softest component in this group making up the face plate.
Fiber optic face plate 27 optically couples the image appearing on the CRT 14 to face plate 28 which is designed to couple the image onto the write side of light valve 12, as is well known. Particular sub-arrays of optical fibers 30 are responsible for coupling corresponding pixels of the image from the face of the CRT onto the light valve. At the same time, the light absorbing fibers 32 are provided to reduce cross talk between sub-arrays. In a typical face plate, of the total fiber content, there may be approximately 3% light absorbing fibers (statistical EMA) necessary to accomplish this. As an alternative, instead of light absorbing fibers 32, face plate 28 may be provided with extramural absorption (EMA) material in the interstitial areas between groups of optical fibers 30. These EMA boundaries which are illustrated in FIG. 4 at 40 serve the same purpose as light absorbing fibers 32, that is, to reduce cross talk between pixels. In this case, the EMA typically makes up about 1% of the total area of fiber optic face plate 28. Like the light absorption fibers 32, the interstitial EMA material 40 is physically harder than any of the components making up optical fibers 30 and extends from one end face of the face plate to its opposite end face.
While fiber optic face plate 28 forming part of overall fiber optic plate arrangement 26 has been described in detail above, it is to be understood that fiber optic face plate 27 also forming part of this overall prior art arrangement may be substantially identical to face plate 28, that is, it will typically include the same lengthwise adjacent optical fibers and either spaced apart light absorption fibers or interstitial EMA material. Therefore, regarding this discussion of prior art, it is to be understood that any comments relating to face plate 28 are also applicable to face plate 27.
Having described the prior art liquid crystal light valve projector assembly and, in particular, the typical fiber optic face plate arrangement for optically coupling its CRT to its light valve, attention is now directed to a particular problem which has heretofore been associated with this type of optical coupling arrangement. More specifically, heretofore, optical face plate arrangements have tended to impart onto the projection plane along with the intended image a faint honeycomb pattern which is know as the "chicken wire effect". Actually, what is imparted onto the projection plane is a visually observable regular boundary pattern corresponding to the cross sectional boundary pattern of certain lengthwise adjacent optical fibers or groups of fibers. This visually observable regular boundary pattern or "chicken wire effect" may extend across the entire projection plane or it may appear in one or more sections of the projection planes. This pattern may extend over the entire face plate and more predominantly around the so-called "multi-multi fiber boundary" of fiber bundles. This fiber bundle boundary pattern 39 is shown diagrammatically in FIG. 5A. The same pattern 39 is also shown in FIG. 2. It is obvious that this phenomenon detracts from the intended image. As will be seen hereinafter, it is the object of the present invention to eliminate or substantially eliminate this particular problem.
Prior to the present invention, it was not entirely understood why the chicken wire effect resulted from the use of the fiber optic plate arrangement in a liquid crystal light valve projector assembly. One reason given heretofore was that the optical fibers making up the arrangement are not perfect. See specifically U.S. Pat. No. 4,917,472 (column 4, lines 30-40). Applicants have since discovered that the degree to which the chicken wire effect exists is at least in large part dependent upon the degree to which the end faces of the fiber optic face plate making up the overall fiber optic plate arrangement are micro-smooth. This is particularly true for the light entering the end face of fiber optic face plate 27, that is, the end face immediately adjacent CRT 14, and the light exiting end face of fiber optic face plate 28, that is, the end face immediately adjacent light valve 12. A highly exaggerated view of a section of face plate 28 is shown in FIGS. 5 and 5B, particularly depicting a segment of its light exiting end face which is generally indicated at 42. End face 42, like the opposite end face of face plate 28 and the end faces of face plate 27 has been polished in a well-known manner.
Applicant has found that using prior art polishing techniques results in an end face which is not particularly smooth in the micro-sense. More specifically, because of the differences in hardness between the light absorbing fibers 32 (the hardest), optical fiber cores 34 (the next hardest), cladding layers 36, and flux material 38 (the softest), there is the tendency to have an irregular surface pattern, as shown in FIGS. 5 and 5B. FIG. 5 illustrates the irregular pattern around a given light absorbing fiber, whereas FIG. 5B illustrates the overall irregular, actually undulating, pattern that extends over the entire surface 42. As seen there, each of the light absorption fibers 32 (because they are the hardest) project further outward from the end face than any other component while the flux material (because it is the softest) defines the greatest recesses within the surface. As a result of this irregular pattern, the light passing across surface 42 (exiting the surface in this case) tends to be reflected and refracted in a way which results in the chicken wire effect discussed above. This is also true for the other end faces not shown in such detail. As will be seen hereinafter, the present invention has end faces that are sufficiently smooth so as to eliminate or substantially eliminate the chicken wire effect otherwise resulting from these micro irregularities.